Method for making subterranean surveys



Dec. 9, 1930. H. c. HAYES METHOD FOR MAKING SUBTERRANEAN SURVEYS FiledMay 15. 1928 3 Sheets-Sheet 1 awumtoz Hervey C Hayes abtomeq 1930. H. c.HAYES 1,784,439

METHOD FOR MAKING SUBTERRANEAN SURVEYS Filed May 15. 1928 3 Sheets-Sheet2 anomcoz V HarvgyC. fla e; 32x Elbtommg wrw Dec. 9,- 1930. H. c. HAYESMETHOD. FOR MAKING SUBTERRANEAN SURVEYS Filed May 15, 1928 3Sheets-Sheet 5 muumnunuumuuuuauuu awuentoz Harvey C HayeS PATNTT METHODFOR MAKING SUBTERRANEAN SURVEYS Application filed may 15, 1928. -SerialNo. 277,968.

It has been a primary object of the present invention to provide a newmethod and arrangement of apparatus for the study of the earths surfaceby geophysical means.

Heretofore, methods of various kinds have been suggested and to acertain extent put into practice, whereby advantage has been taken ofvariations in the different physical properties of the earths crust,such as have been found to take place in accordance with the nature ofthe structure from point to point. Some of these methods have dependedon variations in the gravitational force that are brought about by thepresence of diflerent formations in the earths crust which give rise todifferences in density. Other methods have relied upon differences inthe electrical conductivity of the substances encountered or u onpeculiarities in the magnetic properties 29 0 these substances.

Another property of the materials forming the earths crust, which hasbeen relied upon to 5 me extent within recent years in exploration workof the type under consideration, is

their elasticity which determines to a large extent the speed of travelof sound or similar waves through the earth. Methods of determinatingthe earths structure, which have depended upon this elastic property of30 materials, have been based upon time measurements. The essentialconsideration has been. the speed of travel of sound or similar wavesbetween two known points at one of which the wave is generated and atthe other of which it is detected by suitable means. From the dataobtained over large areas in this way, the presence of formations givingrise to a different speed of travel ofthe waves than normal has beenpredicted. For exam ple, the transmission of a sound wave between twopoints at a greater speed than is normally expected in the upper surfacesoil has been found to indicate the presence of a salt dome in the pathof the wave through v which the latter has been able to travel at aspeed of four or five times that in=the upper surface soil.

The present invention has to do with the transmission of sound orsimilar waves through the earth but entirely different char- ATED ER THEACT OF MARCH 3, 1883, AS AMENDE APRIL 30, 1928; 370 0. G. 757)acteristics or properties of the media through which the waves travelare observed and depended upon to aid in the location of desiredsubterranean formations. The present invention contemplatesconsideration of the coeflicients of absorption and reflection of wavestransmitted through diflerent media. It is well known that thecoeflicient of absorption of a sound or similar wave varies not onlywith the nature of the medium through which the wave is travelling butalso with the particular frequency or the wave as well. Thus a Wave of agiven frequency in travelling through a particular medium will beabsorbed to a certain definite extent, depending upon distancetravelled, and a corersponding decrease in the amplitude of the wavewill be noticeable. On the other hand, the same wave, if passed the samedistance through another medium, will be absorbed to a great er orlesser extent, depending upon the absorption properties of the medium.So, also, iftwo waves of difl'erent frequencies are transmitted througha given medium, each willbe absorbed and have its amplitude decreased ina particular relation, depending upon the coeflicient of absorption foreach frequency. In the same way a wave whichis reflected from areflecting surface in its path will be diminished in intensity andamplitude to an extent depending upon the coeflicient of reflection fromthe particular surface, and this coefiicient will vary in accordancewith the frequency of the wave transmitted.

It is upon this phenomenon of variation in the coefiicients ofabsorption and reflection in accordance with the frequencies of thewaves transmitted, as well as the media of transmission or reflection,that the present invention is based. Toward the end of discoveringstructural variations in the earths surface it is proposed to comparethe amplitudes oi waves of certain determinable frequencies as they aredetected at a plurality of different points at or some distance from thesource of the waves.

The absorption of wave energy, as is well known, is in accordance withthe equation in which 1 stands for the intensity or the fwave at aparticular point of observation, I, is the intensity of the wave at thesource, 8 is the logarithmic base, at is the coeficient of absorption ofa wave of a particular freguency in a particular medium and S is theistance of the point of observation from the source of the wave. It willthus be seen that the intensity of a wave in traveg away from its sourcedoes not vary directly with the distance but in a gerithxnic relationthereto. Furthermore, to intensit a wave is proportional to the squareits amplitude so that in the above equation we may substitute A for lwhere A stands for the amplitude of the wave.

It would be dificult to detere the amplitude or intensity of a wave asit is emitted from a source so as to enable one to determine an thevalue of oz directly from an observation taken at some distance from thesource. For this reason it is contemplated by the present invention toedect a comparison of the values of or without actually determiningthese values. Thus, if a complex wave composed of two or more frequencycomponents is set ,w at a ven point and after travelling a dell" nitevristance is observed by suitable instruments at another point so thatthe amplitudes of the diderent frequency components may be comparedtheir ratios will bear a definite nelation to the original intensities,depen upon the coeficients of absorption of the di ferent frequencies inthe particular medium involved. Now it the same wave could be observedat dider'ent points away from the source but at equal distances from thelatter, the ratios of the plitudes of freuency components should be thesame ii? t e medium of tr in an were the same. Thisis for the reasonthat since the quencies and the medium are the same with relation tbeach of the points of observation, the coeficients of absorption will bethe se, and since the initial intensity is the same for all, and thedistancetravelled is the same, the final intensities, and hence theamplitudes of the several components, would be the so. If it wereossible to construct dein and recording ents exactly alike so l at awave of given intensity would acct all in struments to the se extent, itwould only be necessa to observe a wave of a single frequency. i"?owever, since each instrument its own characteristics as to sensitivity,it he comes desirable to a factor by considering the ratios oi hd'erentuency components received by each cut. Thus while the al amplitude of awave ore given in reproduced on difien ent it would be diderent for theratios of the a :--=plitudes of two waves oi v defiite frequencies d inh as reroduced on the several instents woul '1 be the a .21 2m In otherW0, each in. d. 1'1. L in: in

reproducing a given wave would represent it by a difierent am litude butall waves reproduced by a sing e instrument would be in the sameproportion. Now it will be clear that if the ratios of the amplitudes ofcertain frequency components of a complex wave are the same at a seriesof points equally distant from the source, the transmitting medium mustbe homogeneous or must at least be the same along each of the pathstraversed. On the other hand, if-the ratio is found to be diderent atone or more of the points than at the others, then there is evidence ofa diflerence in the structure of the several wave paths.

As will be explained more fully hereinafter, it is possible also todetermine the actual coeficients of absorption of the media throughwhich the waves are transmitted and to predict the presence and natureof intru sive masses from a study of a single record of a simpleharmonic wave or a single component of a complex wave. v

in carrying out the preferred method, a complex wave, i. e. one havingcomponents of several distinct equenp'ies, is set up at a given sourceby any suitable means, such as b the detonation of an explosive charge.uitable detecting devices may then be stationed at various selectedpoints equally distant from the source and may bev connected to anysuitable form of recording devices which are capable of reproducing theparticular wave forms wliich are being emitted. These records willpreferably beformed pho-. tographically.

It will be found that the record obtained at each point in this way willcontain sevoral impressions of the same wave impulse. For example, oneimpression or reproduction will be made upon the record from that partof the wave impulse which has travelled directly surfacesoil. Anotherreproduction will be made b virtue of-a portion of the wave im-. pulse wich may have travelled downwardly to a 1;: extent and passed through amore elastic medium, sue as a salt dome, for a considerable extent untilby refraction it will be directed toward the same detector. Either inaddition to or instead of the refracted wave, there may be formed animpression of a reflected wave which has travelled downwardly to aconsiderable extent to a reflecting surface and thence to.- ward thedetector. Still another impress on may be created by virtue of a portionof the wave im ulse travelling through the air and strikmg the detectorat a later instant.

Since the approximate speed of travel of the wave through the normalupper. surface will enerally be known, and since the film on which therecord has been formed may be made to travel at a substantially uniformrate, it not be difiicult to distinguish to the detector through theupper received still later.

In connection with the preferred mode of carrying out the invention asexplained, it is only necessary to consider the first impulse which hasbeen recorded at each point or station. The wave train which has beenreproduced from the first impulse may be analyzed by means of any formof harmonic analyzer and the ratio of the amplitudes of anycorresponding wave components at the several stations may be compared.If they are all of the same value it may be assumed that the wave pathsfrom the source to all of the stations are of the same structure whereasif one or more of the ratios differ from the rest, the presence of somesubterranean deposit in the corresponding paths may be predicted. Forexample, in certain areas where the presence of a salt dome is suspectedthe difference in the ratio for one of the paths over that for the otherpaths may serve to indicate the approximate location of a dome. By thentaking a series of similar observations from different angles across theprobable location of the dome, its position and extent may be moredefinitely predicted.

W'ith the foregoing objects and general explanation of the nature of theinvention in view, one suitable form and arrangement of apparatus forcarrying out the novel method will now be described. It should beunderstood, however, that the detailed description which follows is onlyfor the purpose of illustration and that many other forms andarrangements of apparatus may be employed in lieu of those disclosed.The description which, follows is to be considered in conjunction withthe accompanying drawings in which Figure 1 is a diagrammatic showing ofan arrangement of apparatus suitable for the detection of hidden masses.

Figure 2 is a similar showing in elevation of the path of waves from thesource to one of the detectors shown in Figure 1.

Figure 3 is a diagrammatic showing in elevation of the method oflocating hidden bodies or faults by means of reflected waves, and

Figure 4 is a detail of a record which may be formed at arty bf thedetector stations.

Referring now to Figure 1,a location designated by the letter A may beadopted as a central station at or near which the explosion or othersound-producing source will be active. Arranged at suitable points atequal distances from the station A, there may be located a number ofreceiving stations B, C, D, and E. These may be arranged in any suitableway, either in an are on one side of the station A or in the corners ofa square having station A at its center, or around the circumference ofa circle. Any number of receiving stations may be employed, one beingsufiicient but a greater number being preferred, not only to better takeadvantage of a single explosive charge, butalso to provide a betterbasis for the study and comparison of waves transmitted in variousdirections. A distance of about five miles'between' station A and thejreceiving stations will be found very satisfactory, although shorter orlonger distances may be adopted if desired. Suitable radio equipmentwill preferably be provided at each station, although it is notessential to the employment of the present invention. This radioequipment may comprise a transmitting set 10 and a receiving set 11having a common antenna 12 at each of the stations. A switch 13 may thenbe provided to place the antenna in circuit with either the transmitteror receiver, as desired.

Among other things, this equipment may be employed for the purpose ofinter-communication between the stations so that all may be notified asto the time when the explosion is to take place and even to indicate theprecise instant of the explosion. F urthermore, each of the receivingstations may be provided with a radio compass 14 by means of which abearing upon the point of 'explosion may be taken either before or afterthe explosion has taken place. For this purpose the antenna at station Awill be located directly at the point of explosion and a series ofsignals will be sent out to be picked up by the loops of the severalradio compasses. By proper adjustment of the loops until a minimumeffect is produced from the signals, the direction toward the oint ofexplosion may be determined and By comparison of this direction with thereading of, a magnetic compass needle, the true direction with respectto magnetic-north may be determined. Any suitable means may be adoptedfor determining the distances from the several receiving stations to thepoint of explosion. For example, a small preliminary air shot may befired at the point of explosion and the time of travel of the soundthrough the air to each of the receiving stations may be noted. Aftersuitable correction for; the wind velocity and direction, the distancesmay thus be accurately determined from the known velocity of sound inair. Any slight inaccuracies in the location of the stations may becorrected for either by shifting the detectors to place them the samedistance from the source or by taking the differences into considerationin th computations.

After the apparatus at all of the stations below has been properly setup and adjusted as determined by the radio communication present outfrom station A noti ing the receiving stations that a sound wave isabout to be generated. An instant later a switch or key 15 may be closedand by means of current I firom a battery 16, an explosive charge 17,buried referably from ten to twent feet the surface of the earth, will eset 0d. Com ressional waves of a complex nature will e emitted in alldirections from this point and certain of these waves will reach each ofthe detectors 18 located at the several receiving stations. Thesedetectors may he of any desired construction, either of electrical ormechanical design. If electrical, they may either be of some specialmicrophone construction or may embody the princiles of magneto-electricdevices.

In igure 1, an electrical type of detector is conventionally shownandconnections are rovided from the latter to a suitable amplirying andrecording unit 19. This unit may include any suitable number of stagesof vacuum tube amplification from the output side of which connectionsmay be made to any suitable form of oscillograph for photographicallyrecording the waves or impulses received b the detectors. As shown atstation B,in igure 1, a connection may also be rovided from the radioreceiver to the oscilograph for the purpose of recording the instant ofexplosion. The speed of a sound wave through the avera e surface soil inthe region explored being nown, and the distance between the oint ofexplosion and the receiving stations aving been determined, it

is easy to ascertain the instant of explosion from the record producedby the surface wave, it being assumed for this purpose that the recordstrip travels at a uniform rate or that time marks are formed thereon asit is fed.

Figure 2 illustrates a little more clearly the paths followed by twodifferent com lex waves generated upon the explosion o the charge 17.One of these waves or wave trains may follow the straight line path 20through the substantially homogeneous upper surface soil. Another wavetrain may follow the path 21 which carries the sound downwardly at asuitable angle until it strikes some portion of a mass 22 of moreelastic material, such as a salt dome. Upon entering this new medium,the wave will be retracted and may pass along a path substantiallyparallel to the upper surface until it reaches a point where a secondrefraction upon leaving the medmm will direct the wave toward thedetector 18.

A sample of record which might be produced under the conditionsillustrated in Figure 2 is shown in Figure 4. On the photographic recordstrip 23 there is a line 24 images -line will be noted and this servesto indicate the first instant of receipt of that part of the waveimpulse which has followed the course 21. Since the velocity ofcompressional waves through more elastic materials, such as a that ofthe salt dome 22, is far greater than in the average surface soil, therefracted wave will reach the detector in advance of the surface waveeven though its course is of considerably greater length. At 26, asecond sudden break in the line 24 will be noted and this will indicatethe first instant of receipt of the surface wave following-the path 20.Now since the waves generated by the explosion of the charge 17 will beof a complex nature, the wave reproduced in the line 24 will beirregular and will depend upon the nature and phase of the components ofthe wave of different frequencies. Any suitable form of harmonicanalyzer such, for instance, as the Michaelson type,'may be employed forthepurpose of analyzing the-waves beginning at the point 25 to determinethe amplitudes of thecomponents of different frequencies. Ifthe ratiosof the amplitudes of waves of certain definite frequencies are the sameas determined from the records at all of the stations, then it may beassumed that all of the wave'paths are of the same structure and that nolocalized subterranean deposits exist. On the other hand, if one or moreof the ratios differ from those comput-ed for the remaining stations, aprediction may be made as to the location of some intrusivemass in theone or more paths affected.

In the discussion which has thus far been given, it has been assumedthat all that it has] been desired to determine is the presence of somehidden mass and its approximate location. If desired, the actualcoeficient of absorption of a medium may also bedetermined. For thispurpose a series of the receiving stations may be located along astraight line leading radially from a source of waves and the distancesfrom these stations to the source 'may be determined. The

intensity of waves of certain definite frequencies as received at theseveral stations may then be noted and the values of the coeficientsdetermined in accordance with the formulae to be developed presently,Since it is impossible to provide two or more instruinents which willproduce records of precisely the same amplitude in response to a givenwave, the instruments located at the several stations should be mutuallycalibrated. so that a'correction factor may be applied to convert all ofthe results to a common basis. Now if we designate the distances fromthe source to two stations in a line by the characters S and S and thecoefficient of absorption of a wave of a given frequency by (1 we mayform the two following expressions:

where I is the initial intensity of the wave at the source and I and Lare the intensities of the wave after it has travelled the distances Sand S respectively.

Dividing each equation by L, and taking the logarithm of both sides, weobtain the following:

g or log I log I a,S and log I -log I H1181 whence by subtraction weget:

I11= "'G1(LS' S1) Since the values of 1 and I may be determined from theamplitudes of the recorded waves and since the distances S and S may bemeasured, the value of (1 may readily be computed. In the same way thevalues of a a etc. representing the coefficients of ab- 1 sorption'ofwaves of other frequencies in the given medium may likewise be computed.

If all that it is desired to determine is the ratio of two coeflicientsfor different frequencies, it will not be necessary to determine thedistances S and S at all. Thus where the subscripts 1 and 2 refer towaves of different frequencies:

all

so that 5 10;; L-log I log Af-log A," w log I l0g I l A 2 1 1 A =2 loglog W Zlogi log The ratio of coefficients may therefore be determineddirectly from the amplitudesof the two diflerent wave components at thetwo receiving stations.

" ployed for the purpose of determining the nature of a medium when itscoefficient of absorption is determined from actual test.

Furthermore, by the use of the tables compiled as above indicated, or inany other suitable way, advantage may be taken of the two waveimpressions formed on the record to aid in determining the averagecoefficient of absorption of the media through which a refracted wavehas passed. Referring to the record reproduced in Figure 4, it will beclear that the wave impression beginning at the point 26 which resultsfrom that portion of the wave which has travelled through the uppersurface soil ma be analyzed to determine the amplitude 0 any desiredfrequency component. Now, in any given territory where a survey of thecharacter proposed is being conducted, the surface soil will be found tobe substantially uniform and its coeflicient of absorption a for theparticular frequency component measured may be determined from theempirical tables. Therefore, in the expression: lo -log I oc S all ofthe quantities may 0 determined with the exception of The equation mayaccordingly be readily solved for the value of this unknown, it beingunderstood that the distance S is determined and the value of log =log Aor 2 log A is computed from the measured wave amplitude.

By now analyzing the refracted wa e impression beginning at the point 25(Figure4) the amplitude A of the same frequency component after it hasfollowed the path 21 (Figure 2) may be measured. This leaves only thevalue a, of the average coeflicient of absorption unknown in theequation,-

where I has the value determined above from the analysis of the surfacewave depression. From this computed value of the average coefiicient ofabsorption of the path 21, some in- 'dication is given as to the natureand extent of some refracting medium which forms a part of the path.Obviously, the direct comparison of the values of A and A representingthe amplitudes of the same frequency component of the impressionsbeginning at 26 and 25 respectively will also give some indication ofthe two different paths of the waves.

In Figure 3 there is disclosed an arrangement of the stations which maybe adopted for the purpose of detecting a fault line or an intrusivemass in a hidden surface by virtue of the coefiicients of reflection ofvarious waves. For this purpose the several receiving stations willpreferably be placed along a straight line extending radially from pointof explosion and somewhat closer to the latter than under thearrangement illustrated in Figure'l. Upon the explosion of we assumethat the portions 30 and 34: are of such di'fierent nature as to havedifierent coefiicie'nts of reflection, this fact will be brought out inthe records at thestations B andG, due to the unexpected diderence inthe ratio of amplitudes of the selected frequency components of thecomplex wave.

The method of determining "the intensity or amplitude of a ivenfrequency component at the source from the surface impression,wherebythechange in the amplitude due to reflection may be noted as explained inconnection with the refracted waves, will be applicable here as well.

Should the area under survey be of such formation as to present twoseparate reflecting surfaces separated by the thickness of the firstreflecting material, each of the detectors or certain of them willindicate this fact by means of an additional im ression, on the record,of the wave impu se. analysis and study of the wave amplitudes of eachof the reflected wave impressions will serve to indicate the nature ofthe several media.

In connection with all of the foregoing discussion, it should be notedthat no reliance is placed upon the actual s eed of travel of the wavesthrough the di erent media, so

that there is littleoccasion in the conduct of the novel method hereindisclosed to measure time with extreme accuracy. The usual time lineswhich have been found essential t9 the older methods of geophysicalexploration, may be dispensed with under the new method. in thedetermination of the frequencies of the waves reproduced on thephotographic recordythe time element may e gauged with suficientaccuracy from the known s eed of travel of the record strip itself. 0also the determination of distances by means of the time interveningbetween the explosion and the receipt of either the air wave or theupper surface wave may be determined with suficient accuracy from theknown speed of movement of the record strip. Should it be founddesirable in working at closer ranges to provide greater accuracy indetermining the time element for either of the purposes mentioned or forany other reason, any suitable means may 'be employed for'indicatingintervals, such as one one-hundredth (1/100) of a second on the recordstrip.

incense 1. A method of surveying subterranean strata which comprisesgenerating a train of complex compressional waves, reproducing the formof said waves on a record as they reach a point remote from thegenerating source and the analysis of the records ofthe complex wavesinto their components.

2. A method of surveying subterranean formations which comprisesgenerating atrain of complex compressional waves, reproducing the formof said waves on a record as they reach a, point remote from thegenerating source, and measuring the amplitude of components of saidwaves of difierent frequen'cies;

3. A method of surveying subterranean formations which comprisesgenerating a train of complex waves,.and measuring the amplitudeofcomponents of said waves of All difi'erent frequencies at'a distancefrom the generating source.

4. A method of surveying subterraneanformations which comprisesgenerating a train of com lex waves, and measuring the am litude 0components of said waves of di erent frequencies at a plurality ofpoints at a distance from the generating source.

5. A method of surveying subterranean formations which comprisesgenerating a train of complex waves, and measuring the amplitude ofcomponents of said waves of difierent frequencies at the source and at adistance from the generating source.

6. A. method of surveying subterranean formations which comprisesgenerating a series of complex waves near the surface of the earth andanalyzing said Waves into their different components after they havetravelled over a plurality of difi'erent paths.

ill

7. A method. of surveying subterranean tan'ce from the generating sourceand analyzing said record to measure the coeflicient of absorption ofthe meziium through which the waves have travelled.

9. A method of surveying subterranean formationswhioh comprisesgenerating e series of waves in the upper surface of the, earth,reproducing the form of said waves on a record as they reach a point ata mown distance from the enerating source and anelyzin'gseid recor tomeasure the coeficient-of reflection of the medium through which thewaves have travelled.

HARVEY o HAYES.

